Normally, an electrical machine with cryogenic cooling incorporates a superconducting exciting winding secured in a hollow rotor which is a rotary cryostat.
The superconducting state of the exciting winding is maintained by cooling that winding to ultralow temperatures of about 4.5 K. This is done with the aid of a coolant which is usually liquid helium.
The lower the temperature of the superconducting winding (the temperature of the superconductor), the heavier the current and the greater the intensity of the magnetic field the superconducting winding can permit; hence, the higher the efficiency of the electrical machine. An increase in the temperature of the superconducting winding of 1 to 3 degrees results in a considerable drop in the efficiency of the machine.
There are a number of different types of superconducting electrical machines, wherein the superconducting state of the rotary exciting windings is maintained by an immersion of the exciting windings in liquid helium.
There is known an electrical machine with cryogenic cooling, comprising a superconducting exciting winding with busbars connected thereto. The superconducting exciting winding is accommodated in a rotor having a cavity filled with a coolant; the winding is also secured to a shaft of the rotor. The rotor has an axial channel for the supply of the coolant to the superconducting exciting winding, and it is provided at one of the shaft's ends. The rotor shaft is further provided with channels for the removal of the coolant, which extend at both of its ends.
Because the coolant is in a rotating cavity, the liquid helium is subjected to the effects of centrifugal forces. This leads to an increase in the temperature and pressure of the liquid helium, which, in turn, results in the formation of a two-phase mixture of the coolant, and in the increase of the vapor content in that mixture. The higher the peripheral speed of the rotor and the greater its radius, the greater the increase in the temperature and pressure of the liquid helium. An increase in the temperature of the liquid helium and, consequently, in the temperature of the rotary superconducting exciting winding is undesirable because superconductors and superconducting windings manufactured today can perform satisfactorily only at temperature below 5 K.
The machine under review is also disadvantageous in that the temperature of the superconducting winding is determined by the thermal conduction of the contact between the winding and the shaft. The temperature of the superconducting exciting winding increases with an increase in the compressive stress due to the compressive forces exerted by the superconducting winding upon the shaft, which forces are necessary to transmit the torque from the superconducting winding to the drive. The way the superconducting winding is secured in place and cooled makes it hard to ensure a uniform temperature field both surface- and radius-wise. If the internal cavity of the rotor is completely filled with liquid helium it is hard to remove the gaseous phase of the helium from that cavity. There are also heat losses and a rise in the temperature of the coolant, which are due to the friction and compression of the two-phase liquid in the centrifugal force field.